MacOS – Permissions errors thrown, but permissions appear to be correct

Using sudo on the open command doesn't do anything particularly useful -- the actual program still gets run under the user's regular identity. To solve this, make sure TextEdit isn't running, then do:

sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit &
open -a "TextEdit" /etc/sshd_config

The first command runs TextEdit as root; the second tells it to open the file. After this, you should be able to edit & save changes. When you're done, be sure to quit TextEdit (or it'll keep editing files as root).

Alternately, use TextWrangler (or its even-more-capable-but-not-free sibling, BBEdit). It handles saving files as root automatically, as well as having easy ways to edit invisible files. I much prefer it for editing config files and the like.

EDIT: Be sure to get the version of TextWrangler from the Bare Bones Software web site, not the version from the Mac App Store -- the App Store version has the save-as-root feature removed to comply with Apple's rules.

The reason these applications can't write to that files is to do with application sandboxing feature of OS X, which you can read about here if you are interested.

One GUI application that can write to these files is TextWrangler which you can download for free here. There is also a version of TextWrangler in the Mac App Store (MAS) but it doesn't have the ability to write to system files by default (MAS guidelines don't allow applications with those abilities), but you can find an update here that will update the MAS version of TextWrangler so that it can.

Alternatively you can use command line tools to edit these files. For example sudo nano https.conf works okay, and nano is relatively user friendly.